The general goal of these investigations is to elucidate the nature of the mechanism mediating energy-linked transport of K ion across the membranes of rat liver mitochondria. It is proposed that a non- electrogenic mechanism effects a coupled influx of K ion and OH minus, or a coupled exchange of K ion and H ion. In testing this hypothesis, experiments are focusing on the dependence of the energy-linked K42 influx on external pH and K ion concentration. Effects of chemically specific reagents (e.g. sulfhydryl reagents) on rates of K ion influx are also being examined. Some experiments will attempt to determine to what extent other cations may be transported by mechanisms similar to or the same as that mediating energized potassium transport.